In general, in the noise spectrum generated by a ducted antitorque tail rotor and by the resulting flow of air, there can be found lines corresponding to pure sounds at a frequency that is related to the speed of rotation of the rotor, to the number of rotor blades, to the geometrical configuration of the rotor and an air flow deflector, and to the shape and the structure of the duct.
Any rotor rotating in a duct that is fed with air that is turbulent to a greater or lesser extent will generate soundwaves that may be organized or random.
Organized waves constitute that which is commonly referred to as “rotational noise”, which is characterized in the noise spectrum by discrete frequencies (lines) corresponding to the rotary frequencies of the blades, and of the transmission shaft, and to their harmonics and sub-harmonics, or to frequencies that are modulated by angular phase shifting of the blades or of the speed of rotation.
Random waves are characterized in the noise spectrum by high spectral density over a very broad band of frequencies. These random waves generate so-called “broadband” noise.
It is known to use absorbent structures to reduce the propagation of soundwaves emitted by noisy devices such as rotors or motors, such structures comprising a rigid partition, a porous wall, and separator means for placing the porous wall at a determined distance from the rigid partition, with cavities being defined between said porous wall and the rigid partition, the cavities being of height that is determined to maximize absorption of a given frequency in the emitted soundwaves.
So-called “quarter-wave” materials are thus known that present cavities of a height corresponding to one-fourth of the wavelength of the basic frequency that is to be absorbed as a priority. Nevertheless, such materials suffer from a certain number of drawbacks.
In a certain number of applications, and in particular in applications relating to ducted antitorque rotors for helicopters, the audible soundwaves emitted are usually made up both of random waves and of organized waves distributed over a broad band of frequencies, causing known materials to present performance that is not sufficient for effective attenuation of the soundwaves made up in this way under all flying conditions. For example it is necessary to process pure sounds and their harmonics, but it is also necessary to process noise sources that operate over a broad range of speed variation as occurs with aircraft in operation over a temperature range extending from −40° C. to +40° C. The parasitic noise sources that need to be processed are therefore numerous and very diverse.
By way of example, U.S. Pat. No. 6,114,652 describes a method of making acoustic attenuation chambers from a honeycomb structure. The cells have at least two absorbent and porous layers having perforations formed therein by means of a laser. The material constituting the layers is based on polymers and is selected for its properties of absorbing energy at a given radiation frequency of the laser. The layers thus present perforations of different diameters that are distributed differently, in order to optimize acoustic absorption properties.
That document describes an absorbent structure for reducing soundwave propagation that comprises a rigid partition, at least one porous wall, and separation means for placing the porous wall at a predetermined distance from the rigid partition, thereby defining cavities of a given height between said porous wall and said rigid partition.